sinus may not manifest itself in other ways... so only when there's a substantial pressure difference we can feel it.
I have dived with a mild runny nose and took precautions going down slowly but experienced no trouble equalising.
I have also dived without any indication of runny nose or sinus but the moment I went deeper, my whole face was painful. It was scary but after 2-3 times equalising it went away completely... and I continued with the dive. However it was enuff to put me off diving for the rest of the trip (luckily it wasn't a hard-core dive trip). Back in KL went to see the doctor... did an x-ray and saw the infected cavities... he poked me in the nose with a swab and left it there for about 20 minutes before pulling it back out, gave some antibiotics and that was it...
so jim, my first bet is sinus